A general description of the prior art follows.
Electromagnetic interferences from radio noise is generated by a spark discharge between electrodes. It is necessary that materials used for electrodes have low energy loss characteristics while also providing suppression effect for high frequency noise.
The ignition systems of internal combustion engines frequently generate radio noise. Radio noise often adversely affects communications systems such as televisions or radios. The interference problem is compounded as the radio noise is migratory; internal combustion engines having noise generating ignition systems are commonly found in vehicles such as automobiles and as the automobile moves the radio noise generated by the ignition system moves.
Following are the primary causes of the radiation of the radio noise in the ignition system:
(1) spark discharge between electrodes of ignition plugs,
(2) spark discharge between a rotating electrode and the fixed electrodes of a distributor, and
(3) spark discharge at a break-contact of the distributor, accompanied with the make and break motion.
There have been efforts to suppress the radio noise generated by the spark discharge between a rotating electrode and the fixed electrodes of a distributor. Such efforts are hereinafter explained with respect to method (I) through method (V). However, none of these prior methods has achieved a satisfactory and effective suppression of the radio noise generated by the spark discharged between a rotating electrode and the fixed electrodes of a distributor.
METHOD (I): employing a rotating electrode including resistor
In METHOD (I) a resistor is embedded in a rotating electrode. However, in such an assembly stray capacity exists in the electrode in parallel with the resistor. As a consequence, this method has disadvantages. For instance, only a small noise suppression effect at high frequency ranges above 300 MHz is obtained while a large loss in ignition energy through the resistor (about several kilo ohms) occurs. Moreover, the noise suppression effect is small, e.g. only 5-6 dB even at a low frequency range below 200 MHz.
METHOD (II): employing a flame spray coating electrode
In this method, a high resistive film is formed on the surface of the electrode by flame spray coating method. This method has following two disadvantages:
(1) a large loss in ignition energy, due to the high resistive film formed on the surface,
(2) a poor noise suppression effect, for example only 5-5 dB in the frequency range below 200 MHz.
METHOD (III): widening the discharging gap
In this method, each discharging gap between a rotating electrode and fixed electrodes is widened to 1.5-6.4 mm. Advantageously, this provides superior noise suppression effect, for example 15-20 dB.
However, widening the discharge gap between a rotating electrode and fixed electrodes suffers from disadvantages. The disadvantages include an extremely large loss in ignition energy due to a wide discharging gap, and electrode corrosion caused by the corrosive gas affecting metals, such as NO.sub.x, which is generated by the higher discharging voltage between electrodes.
METHOD (IV): employing boride, silicide, carbide and conductive ceramics (with the resistivity of 10.sup.-6 -10.sup.-2 ohm cm) for material of electrodes.
The resistivity of such electrodes is relatively small so that the energy loss can be small. However, this method has disadvantages such as a poor noise suppression effect, for example only 5-10 dB at the frequency range below 300 MHz, and easy consumption of electrodes because the low thermal conductivity of the materials causes the local heating of the electrodes.
METHOD (V): employing conductive ferrite for electrodes
This method provides a good noise suppression effect, for example 10-15 dB. However, the electrode is heated by the large current flowing in inductive discharge between the discharging gap. Also the method has such a disadvantage as the local consumption of the electrode because of the discharge heating. Conversely, employing such ferrite that has a high resistivity, both the noise suppression effect and the durability of the electrodes are sufficient, but the energy loss is seriously large.